What causes the "washboard" effect on unpaved roads?

February 20, 2003

Dear Straight Dope:

We live in a rural area and on many of the unpaved roads a washboard effect develops. Others I've asked an explanation say it's a result of bumps causing tires to leave the ground and then land, and erosion takes care of the rest. This answer doesn't seem to account for varying speeds, car weights and tire sizes. The ruts seem pretty uniform in their depth and spacing--wouldn't erosion create a more haphazard effect?

— mileary

The phenomenon you refer to is known as "washboarding," a wave-like pattern on unpaved roads that might more aptly be called speedbump hell. As you've observed, the ruts occur with striking regularity, belying a chaotic event like erosion.

According to Tom Pettigrew, a Forest Service engineer, the cause is an unlikely source: your car's suspension. (Well, maybe not yours specifically, but it's not innocent in this matter, either.) A vehicle's suspension system distributes the shock and energy of road irregularities with a bouncing rhythm called harmonic oscillation. At each downstroke, the wheels exert extra force on the road, causing the particles in the road to either pack or displace at regular intervals. Once a pattern of ruts starts to establish itself, it becomes self-reinforcing due to what engineers call forced oscillation. The next car hits the same irregularities in the road and bounces at the same rate, causing the pattern to become more and more defined. Forced oscillation overcomes minor variations in oscillation rate that might otherwise arise due to differences in car weight.

Wouldn't variations in speed affect the washboard pattern? Sure, which brings us to another critical part of the feedback loop: you, the driver. Drive too fast on a washboard road and the downstroke exerted by the car wheels may meet the road at a point where a bump is ramping upwards. You know what that means: You bounce off the ceiling. Instinctively most drivers slow to a speed at which the downstrokes coincide with the troughs between bumps, reinforcing the pattern.

Washboarding is inevitable in any unpaved road that sees fairly heavy traffic. The only way to avoid it is to: (a) radically redesign how automotive suspensions are made, (b) give up suspensions altogether, or (c) keep off those dirt roads.

Staff Reports are written by the Straight Dope Science Advisory Board, Cecil's online auxiliary. Though the SDSAB does its best, these columns are edited by Ed Zotti, not Cecil, so accuracywise you'd better keep your fingers crossed.